Engines may be configured with an oxygen sensor coupled to an intake passage for determining the oxygen content of fresh intake air. In particular, the sensor measures the partial pressure of oxygen in the aircharge following equilibration. The aircharge amount may be further corrected for the presence of diluents which can react with oxygen at the sensor, thereby affecting the sensor's output. For example, the oxygen sensor output is corrected for the presence of humidity, hydrocarbons from EGR, purge fuel vapors, crankcase ventilation fuel vapors, etc. One example of such an approach is shown by Surnilla et al. in US patent application 20140251285.
The corrected aircharge estimate can then be used for controlling engine fueling. However, the inventors herein have recognized that oxygen sensors have noise that needs to be filtered to determine a mean signal value. For example, the output of the sensor may be processed using Infinite impulse response (IIR) filtering that is done via analog electronics or via a digital computer algorithm. Such processing adds a delay which reduces the performance potential of controllers using the oxygen sensor data. In addition, the processing is computationally expensive and may not be feasible with current ECU architectures. Further, the signal processing may lose accuracy and/or require even more computation resources during transient operation, e.g., when engine speed is increasing.
The above issues may be at least partly addressed by a method for an engine, comprising: sampling an intake oxygen sensor signal at even increments of time, storing each sampled signal in a buffer, processing the stored sampled signals in the buffer at even increments of engine crank angle, and adjusting an engine operating parameter (such as engine fueling) based on a selected one of the processed sampled signals. In this way, oxygen sensor noise can be removed while minimizing delays.
As an example, during conditions when the engine is operating with one or more of EGR, purge, or crankcase ventilation enabled, a controller may estimate a net oxygen content of the intake aircharge based on the output of an oxygen sensor coupled to an intake passage of the engine. The net oxygen content may not need to be compensated for the presence of diluents such as the purge or crankcase fuel vapors and the EGR. In particular, the inventors have recognized that a catalyzing oxygen sensor measures the net air concentration that needs a matching amount of fuel. Consequently, the air charge estimation based on the output of the oxygen sensor is insensitive to (and therefore independent of) the presence of diluents in the air, allowing for the oxygen sensor to be used as a manifold pressure sensor. The oxygen sensor signal may be sampled at even increments of time and each sampled signal stored in a buffer. Each of the sensor samples may be stamped with the corresponding angle of the crankshaft at time of sampling. When the aircharge is to be computed (e.g., once per firing period), the oxygen sensor samples with an angle stamp corresponding to a desired angle period, such as one firing period into the past (e.g., samples from the immediately previous firing period) are retrieved from the buffer. These signals are averaged and used to compute aircharge for a particular cylinder. Once the aircharge of the cylinder is determined, the proper injection amount of fuel may be computed.
The technical effect of sampling oxygen sensor signals at even increments of time and then processing them in a buffer at even increments of engine crank angle is that the incapacity of existing ECUs to both sample and process sensor signals at fine increments of a crankshaft angle (e.g., 6 crankshaft degrees) is overcome. Using an angle stamp for each sensor signal that is sent to a buffer circumvents the need to have a controller-interrupt at pre-specified angles because each sample does not need to be processed immediately. As such, this allows the processing of a set of signals for a given cylinder firing event to be carried out when the next cylinder firing event is about to take place (for example, every 240° on a three cylinder engine). As a result, accurate estimation of aircharge may be provided using an existing oxygen sensor while minimizing the processing power required for estimating the aircharge. As such, this allows for faster and more accurate engine fueling and torque control, thereby improving engine performance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.